The present invention relates to sampling filters. These filters are similar to digital filters because they use a sampling process, but they differ therefrom because the samples used are not digitized. This similarity permits the use of methods of synthesis used in the computation of digital filters. Such filters are particularly useful in automatic telephone switches of the temporal type.
The manufacture of such filters is known and a description thereof can be found in the article "Transverse Filtering Using Charge-Transfer Devices" published by Dennis D. BUSS et al. in volume SC8, No. 2, of April 1973 in the review "IEEE Journal of Solid-State Circuits".
The charge-transfer devices used in these filters are also known. We will name them according to the notation CCD in English-language documents. A detailed description thereof will be found in the article "Charge-Coupled Devices" published by Gilbert. F. AMELIO in volume 230, No. 2 February 1974 of the review "Scientific American".
The sampling used disturbs the response curve of the filter and causes a phenomena known for example under the name "spectrum folding" or "ghosts". In order to minimize this phenomena, the filter most often used is a circuit of the "sample hold" type which allows a staircase output signal to be obtained whose spectrum is less extended than that of a signal composed of narrow separate pulses.
It is nevertheless often necessary to complete the action of such a filter by analog filtering more or less depending on the results desired. This is particularly the case in telephony where complementary filter lobe suppressors are used when a very considerable attenuation of the spurious signals outside the band is required.